1. Field of the Invention
The present invention relates to dielectric ceramics, manufacturing methods thereof, and multilayer ceramic capacitors formed by using the dielectric ceramics, and more particularly, relates to an improvement by which one is able to advantageously reduce the thickness of a dielectric ceramic layer forming a multilayer ceramic capacitor.
2. Description of the Related Art
A multilayer ceramic capacitor has been generally formed by the following steps.
First, ceramic green sheets containing a powdered starting material for a dielectric ceramic are prepared, each of which has on a surface thereof a conductive material having a desired pattern to be formed into an internal electrode. As the dielectric ceramic, for example, a material containing BaTiO3 as a primary component may be used.
Next, a plurality of ceramic green sheets including the ceramic green sheets provided with the conductive material described above are laminated to each other and then processed by thermo-compression bonding, thereby forming an integrated green laminate.
This green laminate is then fired, and hence a fired laminate can be obtained. Inside the laminate thus obtained, internal electrodes composed of the aforementioned conductive material are provided.
Subsequently, external electrodes are formed on external surfaces of the laminate so as to be electrically connected to the respective internal electrodes. The external electrodes are formed, for example, by applying a conductive paste containing a powdered conductive metal and a glass frit on the external surfaces of the laminate, followed by firing.
As described above, the multilayer ceramic capacitor can be formed.
As a conductive material used for the internal electrodes described above, for example, a relatively inexpensive base metal such as nickel or copper has been increasingly used in recent years in order to reduce a manufacturing cost of a multilayer ceramic capacitor as low as possible. However, when a multilayer ceramic capacitor having internal electrodes composed of a base metal is formed, firing must be performed in a neutral or a reducing atmosphere in order to prevent the base metal from being oxidized, and hence a dielectric ceramic used for a multilayer ceramic capacitor must have reduction resistance.
As a dielectric ceramic which has reduction resistance as described above and which can form a multilayer ceramic capacitor having a capacitance-temperature characteristic which satisfies the B characteristic specified by the Japanese Industrial Standards (JIS), a ceramic has been used in practice which is composed, for example, of BaTiO3 as a primary component; an oxide of a rare earth element; an oxide of manganese (Mn), iron (Fe), nickel (Ni), copper (Cu) or the like; and a firing auxiliary agent.
As the dielectric ceramic described above, a dielectric ceramic composition having a high dielectric constant and a long life under a high temperature load condition has been proposed, for example, in Japanese Unexamined Patent Application Publication Nos. 5-9066, 11-302071, and 2000-58377.
In addition, a dielectric ceramic having a so-called core-shell structure has been proposed in Japanese Unexamined Patent Application Publication Nos. 6-5460, 2001-220224, and 2001-230149 in view of the structure and texture of the dielectric ceramic.
In addition a dielectric ceramic having a high dielectric constant and superior insulating properties has been proposed which is obtained by controlling a grain boundary structure of the ceramic in Japanese Unexamined Patent Application Publication No. 9-270366.
In Japanese Unexamined Patent Application Publication No. 10-74666, a dielectric ceramic having superior reliability has been proposed which is obtained by controlling the size of segregation phases of the ceramic.
Concomitant with the recent advance of electronic technology, there has been an increasing trend toward miniaturization of electronic components in general, and miniaturization and larger capacitance of multilayer ceramic capacitors have also been actively pursued. As an effective method for realizing the miniaturization and larger capacity of multilayer ceramic capacitors, for example, a reduction in thickness of a dielectric ceramic layer may be mentioned. The thickness of a dielectric ceramic layer has been reduced to about 2 μm or less on a mass production level and to about 1 μm or less on an experimental level.
However, although the dielectric ceramic disclosed in Japanese Unexamined Patent Application Publication No. 5-9066 exhibits superior electrical insulating properties, when the thickness of the dielectric ceramic layer is reduced, for example, to 5 μm or less, and in particular, to 3 μm or less, the reliability of the dielectric ceramic cannot always sufficiently satisfy the requirements of the marketplace.
As is the case described above, as the thickness of the dielectric ceramic layer is reduced to 2 μm or less, the capacitance-temperature characteristic and the reliability of the dielectric ceramics disclosed in Japanese Unexamined Patent Application Publication Nos. 11-302071 and 2000-58377, are disadvantageously degraded.
In addition, the reliability is also disadvantageously decreased in the dielectric ceramic having a so-called core-shell structure disclosed in Japanese Unexamined Patent Application Publication Nos. 6-5460, 2001-220224, and 2001-230149, as the thickness of the dielectric ceramic layer is reduced. In the dielectric ceramic disclosed in Japanese Unexamined Patent Application Publication No. 9-270366, reaction between the primary component and the additive component is unstably carried out since an additive component added to a primary component such as BaTiO3 is melted in a firing step, and as a result, in particular, when the thickness of the dielectric ceramic layer is reduced, a problem may occur in that the reliability cannot be stably ensured.
The dielectric ceramic disclosed in Japanese Unexamined Patent Application Publication No. 10-74666 also has a problem in that when the thickness of the dielectric ceramic layer is reduced, and in particular, is reduced to 3 μm or less, the reliability is decreased. In addition, since large amounts of volatile components such as lithium (Li) and boron (B) are present, a problem of the stability of electrical properties including the reliability occurs when the thickness of the dielectric ceramic layer is reduced.
When the thickness of the dielectric ceramic layer is reduced in order to meet the trend toward the miniaturization and the higher capacitance of a multilayer ceramic capacitor and the rated DC voltage is set as same as before, the applied electric field per layer of the dielectric ceramic layer is increased, and as a result, the reliability is significantly decreased.
Accordingly, a dielectric ceramic has been increasingly desired which can realize a reliable multilayer ceramic capacitor even when the thickness of a dielectric ceramic layer thereof is reduced.